This invention relates to an electron capture detector adapted to be used for a gas chromatograph.
Among the different kinds of detectors used for a gas chromatograph, electron capture detectors are useful for the detection of compounds tending to combine with electrons, such as halogens and nitro compounds. For this reason, electron capture detectors are commonly used for the detection of residual amounts of organic mercury, agricultural chemicals, and PCBS, as well as very small amounts of steroids and amino acids, by converting them into a derivative with a tendency to combine with electrons.
To briefly explain the principles of operation of an electron capture detector, a radioactive isotope such as .sup.63 Ni is sealed inside a detection cell, and a carrier gas is introduced such that its molecules are ionized by the radiation from the isotope, free electrons being thereby emitted. If a voltage is then applied to a positive electrode disposed inside this detection cell, a certain current will flow through the electrode due to these free electrons under a steady condition. If a pulsed voltage is applied to the electrode and the difference between the pulse current which flows through the electrode as a result of this pulsed voltage and a specified current I.sub.S is inputted to an integrator, the output from this integrator is a voltage which depends on the difference between the average value of the pulse current (the pulse current per unit time) and the specified current I.sub.S. If this output from the integrator is introduced into a voltage-frequency converter to obtain therefrom a pulse signal with frequency depending on the difference between these two currents and a pulse voltage is generated based thereon, the frequency f of the pulse voltage applied to the electrode in a steady state takes a value depending on the specified current I.sub.S.
If molecules of an electron-capturing substance are introduced into the detection cell, these molecules serve to capture the free electrons emitted from the carrier gas, causing the density of the free electrons to be reduced. Since the negative ions which have captured the free electrons move much more slowly than the free electrons, the current which flows through the electrode also diminishes due to this decrease in the density of the free electrons. This causes the output voltage from the integrator to grow larger, and the frequency f of the pulse signal outputted from the voltage-frequency convertor increases. In other words, the number of pulses which are generated per unit time increases so as to make up for the decrease in the number of electrons captured by each pulsed voltage.
It is known that the density a of an electron-capturing substance and the pulse frequency, f are related as follows: EQU Kf=(k.sub.1 a+K.sub.D) (1)
where Kf, k.sub.1, and K.sub.D are constants. (See, for example, R. J. Maggs, et al.; "The Electron Capture Detector--A Mode of Operation", Analytical Chemistry, Vol. 43, No. 14, December (1971) 1967.) Thus, the change in the pulse frequency .DELTA.f from the situation where a=0 (that is, where no electron-capturing substance has been introduced into the detection cell) is proportional to the density a, being given by: EQU .DELTA.f=(k.sub.1 a+K.sub.D)/K-K.sub.D /K=(k.sub.1 /K)a (2)
In other words, the density a of the sample can be calculated from the change in the frequency .DELTA.f as follows: EQU a=.DELTA.f/(k.sub.1 /K) (3)
where the value of (k.sub.1 /K) is understood to be preliminarily determined experimentally. Since the output voltage V from the integrator depends on the change in the pulse frequency f, a chromatogram of a target electron-capturing substance can be obtained by recording this output voltage V with an elapse of time.
As measurements are repeated and the electrode and the interior of the detection cell become polluted by samples, it becomes difficult for the current to flow and the frequency f tends to increase. In other words, even if a same sample is measured, the result of measurement will tend to change with time. Another problem which is associated with the use of a radioactive isotope is that the interior of the detection cell cannot be washed easily because it requires a special technology.